1. Field of the Invention
The present invention relates to a solenoid-operated flow control valve which is particularly suitable for use in a hydraulic system wherein a working fluid contaminated by and laden with minute particles of metallic materials is circulated.
2. Description of the Prior Art
Solenoid valves are widely used in various hydraulic systems to electronically control flow of a fluid.
As shown in FIG. 1A, a solenoid valve may typically include a movable valve member 1 connected to an armature or plunger 2. The armature 2 is operated by a solenoid assembly comprised of a solenoid windings 3, a magnetic pole piece 4, and a yoke member 5. The armature is biased downwards by a return coil spring 6 which is supported at its upper end by a spring retainer 7 which may be in the form of an adjusting screw adjustably screwed into the pole piece 4. An annular spacer 8 made of a non-magnetizable material is fixed to the lower end of the pole piece 4 so as to limit the upward travel of the armature 2 to thereby space the armature at a given distance from the pole piece in the fully open position of the valve.
In the case where the solenoid valve is controlled by an electronic control system, it is customary to operate the solenoid valve on the duty cycle basis by cyclically energizing the solenoid windings with intermittent drive pulses having a frequency which may range, for example, from 200 to 300 cycles per second, the degree of opening of the valve being controlled by varying the width, or duty factor, of respective drive pulses.
Such an intermittent energization of the solenoid coil would result undesirable chattering of the valve. In order to suppress or subdue chattering of the valve that would result from the duty cycle operation of the solenoid, the armature chamber 9 receiving the armature 2 is filled with a fluid to thereby damp the vibratory movement of the armature 2. To this end, the armature chamber 9 is communicated by an annular passage 10 with the outlet port 11 of the valve to admit the fluid at the outlet to flow into the armature chamber, the passage 10 being formed between the armature and the yoke member 5. The fluid in the armature chamber is drained by a drain passage 12 which extends axially throughout the spring retainer 7.
In certain applications of the solenoid valves, a hydraulic fluid is inevitably contaminated by fine particles of ferrous materials resulting from wear of machine parts. For example, in an automatic transmission system of a vehicle, an automatic transmission fluid is circulated through various metallic moving parts such as gear trains and clutch discs so that the fluid will become considerably contaminated by finely divided debris, fragments or particles of ferrous materials resulting from wearing of gears and other metallic parts.
The problem encountered with the solenoid valves as used to control a ferrous contaminant-laden fluid is that ferrous particles born in the hydraulic fluid are magnetically attracted and trapped in the magnetic gaps of the solenoid structure as the fluid is passed through the armature chamber.
More specifically, as shown in FIG. 1B wherein parts and members encircled by a circle in FIG. 1A are shown in an enlarged scale, ferrous particles are attracted to and deposit on the lower end face of the pole piece 4 as well as on the upper end face of the armature 2 as schematically shown at 12 and 13. Metallic particles are also magnetically held at the radial gap between the armature and the yoke member as shown at 14. Furthermore, particles are accumulated between the consecutive turns of the coil spring as shown at 15.
The ferrous particles magnetically accumulated in this manner at the magnetic gaps of the solenoid will be oriented along the magnetic flux path in an acicular fashion to project from one surface toward the opposite surface of the magnetic gap, thereby giving rise to a situation in which the gap is somewhat bridged or short-circuited by chains of attracted particles. As a result, the magnetic permeability across the gap is inadvertently increased in response to a lapse of time so that the operating property of the solenoid valve, e.g., the current versus fluid pressure characteristics, is undesirably altered during the service life of the solenoid valve.
Accordingly, it is an object of the present invention to provide a solenoid valve which is suitable for use in controlling a fluid which is contaminated by and laden with minute particles of metallic materials.
Another object of the invention is to provide a solenoid valve which is capable of effectively preventing ferrous particles from accumulating at the magnetic gaps of the solenoid assembly.
A still another object of the invention is to provide a solenoid valve which exhibits a constant operating characteristics throughout the service life of the valve.